Did not work aoColumns mystring when using sAjaxSource

I used mystring for null last sorting, but when I changed to use sAjaxSource, it didn't work.
In server side php code, server_processing.php, I could change ordering by sql ORDER BY function, so is it possible to add new column with number of records?

I have the record more then 100000, which is suitable sorting method, sql or aaSorting?